Fix wrong posttype

The ```get_post_type_in_lang()``` method can return a boolean instead of a post type but the return value is always use to set the post type. When a post type like **nav_menu_item** is called it can't be found in ```$this->lang_map2``` which results that ```get_post_type_in_lang()``` returns false which mean in ```WP_Query``` that the post type is post and not **nav_menu_item**.

Also replaced ```apply_filters( 'bbl_translated_post_type', true, $post_type )``` to ```bbl_is_translated_post_type()``` which is exactly the same.